1. Field of the Invention
This invention relates to devices for measuring the emittance or reflectance of a surface and, more particularly, for measuring the relative emittance or reflectance of an arbitrarily sized plane or curved surface with respect to a pair of known standards.
2. Description of the Prior Art
When electromagnetic energy strikes a material the electromagnetic energy is either reflected from the surface of the material, absorbed by the material or transmitted through the material. For opaque materials none of the incident energy is transmitted through the material so that the ratio of energy reflected (the reflectance .rho.) plus the ratio of the energy absorbed by the material (the absorptance .alpha.) always equals unity. .alpha. + .rho. = 1. For transparent or translucent materials the ratio of the energy transmitted (the transmittance, .tau.) plus the reflectance and the absorptance always equals unity. .alpha. + .rho. + .tau. = 1. All materials emit electromagnetic energy, and the intensity of the electromagnetic energy emitted is a function of the temperature of the material. The relative ability of a material to emit electromagnetic energy is termed "emittance, .epsilon. " which is the ratio of the radiant energy emitted by the object and the radiant energy emitted by a perfect radiator or "black body" at the same temperatures. By Kirchhoff's law when an object is at thermal equilibrium the absorptance is equal to the emittance. .epsilon. = .alpha. In other words, a surface which easily absorbs energy, such as a darkly colored surface, also easily emits energy. On the other hand, a surface which has a low absorptance, such as a shiny metallic surface, also has a low emittance. Since the emittance, reflectance and absorptance of an object are related functions, a measurement of one physical property is easily translated into a measurement of the remaining physical properties.
In military or civilian manufacturing and procurement it is often specified that the surface of an object must have a specific narrow emittance range. The emittance may be selected by such techniques as painting, electroplating, taping, coating and machining. In the design of an apparatus suitable for measuring temperatures, in spacecraft coatings used to control temperatures, and generally in any apparatus where thermal conditions are influenced by heat transfer through radiation, it is essential to know and control the emittance of various surfaces involved.
Various types of devices for measuring emittance or reflectance are available on the market. Most of these devices are generally relatively inaccurate, not conveniently portable, are complex and time consuming to set up for use, and often require an extended period of time to complete the measurement. Furthermore, many of these devices are only capable of measuring the emittance of planar surfaces and thus are not suitable for measuring the surface emittance or reflectance of many products. The most common variety of devices for measuring the emittance of a surface performs the measurement by heating the object under test to a predetermined temperature and measuring the emittance by utilizing the known relation that the energy emitted is proportional to the emittance times the fourth power of the temperature of the test sample. Such devices are incapable of making a measurement until the temperature of the object has stabilized. As a result a relatively long period is often required to make the measurement, and measurements on large objects are sometimes impossible since too much heat is often dissipated for their temperatures to stabilize at a relatively high value. Furthermore, a lot of these current devices require additional quantitative measurements of the temperature of the surface under test or some other reference surface which requires additional circuit complexities, and could also cause damage to the surface under test.